Problem with mirroring (mirrored part is higher than the parent object and a middle object)
If you don't want to use mirror modifier, you can create linked duplicate of your object. Select the fender mesh, press Alt+D and click left. Now your duplicated object is at the same position as the original object. Now set the x-scale of your duplicate object to -1. Since the second object is not just duplicated, but linked, all mesh edits will show on both objects. This is also called an instance. You can make many instances of the same object.